U.S. patent application number 16/548237 was filed with the patent office on 2020-02-27 for vehicle brake system with brake fade detection.
This patent application is currently assigned to ZF Active Safety US Inc.. The applicant listed for this patent is ZF Active Safety US Inc.. Invention is credited to Wenbo Yu.
Application Number | 20200062233 16/548237 |
Document ID | / |
Family ID | 69412976 |
Filed Date | 2020-02-27 |
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United States Patent
Application |
20200062233 |
Kind Code |
A1 |
Yu; Wenbo |
February 27, 2020 |
VEHICLE BRAKE SYSTEM WITH BRAKE FADE DETECTION
Abstract
A method of detecting brake fade for a vehicle brake system
having wheel brakes susceptible to brake fade, includes the steps
of: (a) providing a brake system including wheel brakes; and (b)
determining the presence of brake fade within the vehicle system
utilizing adaptive references maps, and wherein the adaptive
reference maps include storing vehicle deceleration data and brake
system fluid volume at different pressure points in a
histogram.
Inventors: |
Yu; Wenbo; (Canton,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ZF Active Safety US Inc. |
Livonia |
MI |
US |
|
|
Assignee: |
ZF Active Safety US Inc.
Livonia
MI
|
Family ID: |
69412976 |
Appl. No.: |
16/548237 |
Filed: |
August 22, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62722649 |
Aug 24, 2018 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B60T 17/221 20130101;
B60T 13/146 20130101; B60T 13/662 20130101; B60T 13/686 20130101;
B60T 7/042 20130101; B60T 13/745 20130101 |
International
Class: |
B60T 17/22 20060101
B60T017/22; B60T 13/74 20060101 B60T013/74 |
Claims
1. A method of detecting brake fade for a vehicle brake system
having wheel brakes susceptible to brake fade, the method
comprising: (a) providing a brake system including wheel brakes;
and (b) determining the presence of brake fade within the vehicle
system utilizing adaptive references maps, and wherein the adaptive
reference maps include storing vehicle deceleration data and brake
system fluid volume at different pressure points in a
histogram.
2. The method of claim 1, wherein step (b) includes establishing an
adaptive reference map of vehicle deceleration at different
pressure points utilizing a histogram by providing a searching
algorithm seeking pre-defined pressure points during a braking
event, and wherein the current vehicle deceleration data is learned
to the reference map of vehicle deceleration when the boost
pressure is within the range of one of the points.
3. The method of claim 2 further determining a vehicle deceleration
factor by measuring the decrease in deceleration during a braking
event, and calculating a change factor in percentage form.
4. The method of claim 3, wherein step (b) includes establishing an
adaptive reference map of brake system fluid volume at different
pressure points utilizing a histogram by providing a searching
algorithm seeking pre-defined pressure points during a braking
event, and wherein the current brake system fluid volume is learned
to the reference map of vehicle deceleration when the boost
pressure is within the range of one of the points.
5. The method of claim 4 further determining a PV factor by
measuring the increase in fluid volume during a braking event, and
calculating a change factor in percentage form.
6. The method of claim 5 further including determining an overall
brake factor by: (a) weighting the vehicle deceleration factor in
percentage form; (b) weighting the PV factor in percentage form;
and (c) adding the vehicle deceleration factor and the PV factor
together.
7. The method of claim 6 further including multiplying the vehicle
deceleration factor and the PV factor by a weighing constant.
8. The method of claim 1, wherein the brake system provided in step
(a) includes a plunger assembly having a housing defining a bore
therein, wherein the plunger assembly includes a piston slidably
disposed therein such that movement of the piston pressurizes a
pressure chamber when the piston is moved in a first direction, and
wherein the pressure chamber of the plunger assembly is in fluid
communication with an output, and wherein the plunger assembly
further includes an electrically operated linear actuator for
moving the piston within the bore.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates in general to vehicle braking
systems. Vehicles are commonly slowed and stopped with hydraulic
brake systems. These systems vary in complexity but a base brake
system typically includes a brake pedal, a tandem master cylinder,
fluid conduits arranged in two similar but separate brake circuits,
and wheel brakes in each circuit. The driver of the vehicle
operates a brake pedal which is connected to the master cylinder.
When the brake pedal is depressed, the master cylinder generates
hydraulic forces in both brake circuits by pressurizing brake
fluid. The pressurized fluid travels through the fluid conduit in
both circuits to actuate brake cylinders at the wheels to slow the
vehicle.
[0002] Base brake systems typically use a brake booster which
provides a force to the master cylinder which assists the pedal
force created by the driver. The booster can be vacuum or
hydraulically operated. A typical hydraulic booster senses the
movement of the brake pedal and generates pressurized fluid which
is introduced into the master cylinder. The fluid from the booster
assists the pedal force acting on the pistons of the master
cylinder which generate pressurized fluid in the conduit in fluid
communication with the wheel brakes. Thus, the pressures generated
by the master cylinder are increased. Hydraulic boosters are
commonly located adjacent the master cylinder piston and use a
boost valve to control the pressurized fluid applied to the
booster.
[0003] Braking a vehicle in a controlled manner under adverse
conditions requires precise application of the brakes by the
driver. Under these conditions, a driver can easily apply excessive
braking pressure thus causing one or more wheels to lock, resulting
in excessive slippage between the wheel and road surface. Such
wheel lock-up conditions can lead to greater stopping distances and
possible loss of directional control.
[0004] Advances in braking technology have led to the introduction
of Anti-lock Braking Systems (ABS). An ABS system monitors wheel
rotational behavior and selectively applies and relieves brake
pressure in the corresponding wheel brakes in order to maintain the
wheel speed within a selected slip range to achieve maximum braking
force. While such systems are typically adapted to control the
braking of each braked wheel of the vehicle, some systems have been
developed for controlling the braking of only a portion of the
plurality of braked wheels.
[0005] Electronically controlled ABS valves, comprising apply
valves and dump valves, are located between the master cylinder and
the wheel brakes. The ABS valves regulate the pressure between the
master cylinder and the wheel brakes. Typically, when activated,
these ABS valves operate in three pressure control modes: pressure
apply, pressure dump and pressure hold. The apply valves allow
pressurized brake fluid into respective ones of the wheel brakes to
increase pressure during the apply mode, and the dump valves
relieve brake fluid from their associated wheel brakes during the
dump mode. Wheel brake pressure is held constant during the hold
mode by closing both the apply valves and the dump valves.
[0006] To achieve maximum braking forces while maintaining vehicle
stability, it is desirable to achieve optimum slip levels at the
wheels of both the front and rear axles. During vehicle
deceleration different braking forces are required at the front and
rear axles to reach the desired slip levels. Therefore, the brake
pressures should be proportioned between the front and rear brakes
to achieve the highest braking forces at each axle. ABS systems
with such ability, known as Dynamic Rear Proportioning (DRP)
systems, use the ABS valves to separately control the braking
pressures on the front and rear wheels to dynamically achieve
optimum braking performance at the front and rear axles under the
then current conditions.
[0007] A further development in braking technology has led to the
introduction of Traction Control (TC) systems. Typically, valves
have been added to existing ABS systems to provide a brake system
which controls wheel speed during acceleration. Excessive wheel
speed during vehicle acceleration leads to wheel slippage and a
loss of traction. An electronic control system senses this
condition and automatically applies braking pressure to the wheel
cylinders of the slipping wheel to reduce the slippage and increase
the traction available. In order to achieve optimal vehicle
acceleration, pressurized brake fluid is made available to the
wheel cylinders even if the master cylinder is not actuated by the
driver.
[0008] During vehicle motion such as cornering, dynamic forces are
generated which can reduce vehicle stability. A Vehicle Stability
Control (VSC) brake system improves the stability of the vehicle by
counteracting these forces through selective brake actuation. These
forces and other vehicle parameters are detected by sensors which
signal an electronic control unit. The electronic control unit
automatically operates pressure control devices to regulate the
amount of hydraulic pressure applied to specific individual wheel
brakes. In order to achieve optimal vehicle stability, braking
pressures greater than the master cylinder pressure must quickly be
available at all times.
[0009] Brake systems may also be used for regenerative braking to
recapture energy. An electromagnetic force of an electric
motor/generator is used in regenerative braking for providing a
portion of the braking torque to the vehicle to meet the braking
needs of the vehicle. A control module in the brake system
communicates with a powertrain control module to provide
coordinated braking during regenerative braking as well as braking
for wheel lock and skid conditions. For example, as the operator of
the vehicle begins to brake during regenerative braking,
electromagnet energy of the motor/generator will be used to apply
braking torque (i.e., electromagnetic resistance for providing
torque to the powertrain) to the vehicle. If it is determined that
there is no longer a sufficient amount of storage means to store
energy recovered from the regenerative braking or if the
regenerative braking cannot meet the demands of the operator,
hydraulic braking will be activated to complete all or part of the
braking action demanded by the operator. Preferably, the hydraulic
braking operates in a regenerative brake blending manner so that
the blending is effectively and unnoticeably picked up where the
electromagnetic braking left off. It is desired that the vehicle
movement should have a smooth transitional change to the hydraulic
braking such that the changeover goes unnoticed by the driver of
the vehicle.
[0010] Brake systems may also include autonomous braking
capabilities such as adaptive cruise control (ACC). During an
autonomous braking event, various sensors and systems monitor the
traffic conditions ahead of the vehicle and automatically activate
the brake system to decelerate the vehicle as needed. Autonomous
braking may be configured to respond rapidly in order to avoid an
emergency situation. The brake system may be activated without the
driver depressing the brake pedal or even if the driver fails to
apply adequate pressure to the brake pedal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a schematic illustration of a first embodiment of
a brake system.
[0012] FIG. 2 is an enlarged schematic illustration of the plunger
assembly of the brake system of FIG. 1.
[0013] FIG. 3 is a graphical representation of an example of
histogram data for use in an algorithm to determine brake fade.
[0014] FIG. 4 is a graphical representation of PV characteristics
for use in PV factor calculation.
[0015] FIG. 5 is a graphical representation of fluid boil PV
characteristics.
[0016] FIG. 6 is a graphical representation of a brake pedal feel
curve comparing normal braking versus braking during a brake fade
event.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] Referring now to the drawings, there is schematically
illustrated in FIG. 1 a first embodiment of a vehicle brake system,
indicated generally at 10. The brake system 10 is a hydraulic
braking system in which fluid pressure from a source is operated to
apply braking forces for the brake system 10. The brake system 10
may suitably be used on a ground vehicle such as an automotive
vehicle having four wheels. Furthermore, the brake system 10 can be
provided with other braking functions such as anti-lock braking
(ABS) and other slip control features to effectively brake the
vehicle, as will be discussed below. In the illustrated embodiment
of the brake system 10, there are four wheel brakes 12a, 12b, 12c,
and 12d. The wheel brakes 12a, 12b, 12c, and 12d can have any
suitable wheel brake structure operated by the application of
pressurized brake fluid. The wheel brakes 12a, 12b, 12c, and 12d
may include, for example, a brake caliper mounted on the vehicle to
engage a frictional element (such as a brake disc) that rotates
with a vehicle wheel to effect braking of the associated vehicle
wheel. The wheel brakes 12a, 12b, 12c, and 12d can be associated
with any combination of front and rear wheels of the vehicle in
which the brake system 10 is installed. A diagonally split brake
system is illustrated such that the wheel brake 12a is associated
with the left rear wheel, the wheel brake 12b is associated with
the right front wheel, the wheel brake 12c is associated with the
left front wheel, and the wheel brake 12d is associated with the
right rear wheel. Alternatively for a vertically split system, the
wheel brakes 12a and 12b may be associated with the front wheels,
and the wheel brakes 12c and 12d may be associated with the rear
wheels.
[0018] The brake system 10 includes a brake pedal unit, indicated
generally at 14, a pedal simulator 16, a plunger assembly,
indicated generally at 18, and a reservoir 20. The reservoir 20
stores and holds hydraulic fluid for the brake system 10. The fluid
within the reservoir 20 is preferably held at or about atmospheric
pressure but may store the fluid at other pressures if so desired.
The brake system 10 may include a fluid level sensor (not shown)
for detecting the fluid level of the reservoir 20. Note that in the
schematic illustration of FIG. 1, conduit lines may not be
specifically drawn leading to the reservoir 20 but may be
represented by conduits ending and labelled with T1, T2, or T3
indicating that these various conduits are connected to one or more
tanks or sections of the reservoir 20. Alternatively, the reservoir
20 may include multiple separate housings. As will be discussed in
detail below, the plunger assembly 18 of the brake system 10
functions as a source of pressure to provide a desired pressure
level to the wheel brakes 12a, 12b, 12c, and 12d during a typical
or normal brake apply. Fluid from the wheel brakes 12a, 12b, 12c,
and 12d may be returned to the plunger assembly 18 and/or diverted
to the reservoir 20.
[0019] The brake system 10 includes an electronic control unit
(ECU) 22. The ECU 22 may include microprocessors. The ECU 22
receives various signals, processes signals, and controls the
operation of various electrical components of the brake system 10
in response to the received signals. The ECU 22 can be connected to
various sensors such as pressure sensors, travel sensors, switches,
wheel speed sensors, and steering angle sensors. The ECU 22 may
also be connected to an external module (not shown) for receiving
information related to yaw rate, lateral acceleration, longitudinal
acceleration of the vehicle such as for controlling the brake
system 10 during vehicle stability operation. Additionally, the ECU
22 may be connected to the instrument cluster for collecting and
supplying information related to warning indicators such as an ABS
warning light, a brake fluid level warning light, and a traction
control/vehicle stability control indicator light.
[0020] The brake system 10 further includes first and second
isolation valves 30 and 32. The isolation valves 30 and 32 may be
solenoid actuated three way valves. The isolation valves 30 and 32
are generally operable to two positions, as schematically shown in
FIG. 1. The first and second isolation valves 30 and 32 each have a
port in selective fluid communication with an output conduit 34
generally in communication with an output of the plunger assembly
18, as will be discussed below. The first and second isolation
valves 30 and 32 also includes ports that are selectively in fluid
communication with conduits 36 and 38, respectively, when the first
and second isolation valves 30 and 32 are non-energized, as shown
in FIG. 1. The first and second isolation valves 30 and 32 further
include ports that are in fluid communication with conduits 40 and
42, respectively, which provide fluid to and from the wheel brakes
12a, 12b, 12c, and 12d.
[0021] In a preferred embodiment, the first and/or second isolation
valves 30 and 32 may be mechanically designed such that flow is
permitted to flow in the reverse direction (from conduit 34 to the
conduits 36 and 38, respectively) when in their de-energized
positions and can bypass the normally closed seat of the valves 30
and 32. Thus, although the 3-way valves 30 and 32 are not shown
schematically to indicate this fluid flow position, it is noted
that that the valve design may permit such fluid flow. This may be
helpful in performing self-diagnostic tests of the brake system
10.
[0022] The system 10 further includes various solenoid actuated
valves (slip control valve arrangement) for permitting controlled
braking operations, such as ABS, traction control, vehicle
stability control, and regenerative braking blending. A first set
of valves includes a first apply valve 50 and a first dump valve 52
in fluid communication with the conduit 40 for cooperatively
supplying fluid received from the first isolation valve 30 to the
wheel brake 12a, and for cooperatively relieving pressurized fluid
from the wheel brake 12a to a reservoir conduit 53 in fluid
communication with the reservoir 20. A second set of valves
includes a second apply valve 54 and a second dump valve 56 in
fluid communication with the conduit 40 for cooperatively supplying
fluid received from the first isolation valve 30 to the wheel brake
12b, and for cooperatively relieving pressurized fluid from the
wheel brake 12b to the reservoir conduit 53. A third set of valves
includes a third apply valve 58 and a third dump valve 60 in fluid
communication with the conduit 42 for cooperatively supplying fluid
received from the second isolation valve 32 to the wheel brake 12c,
and for cooperatively relieving pressurized fluid from the wheel
brake 12c to the reservoir conduit 53. A fourth set of valves
includes a fourth apply valve 62 and a fourth dump valve 64 in
fluid communication with the conduit 42 for cooperatively supplying
fluid received from the second isolation valve 32 to the wheel
brake 12d, and for cooperatively relieving pressurized fluid from
the wheel brake 12d to the reservoir conduit 53. Note that in a
normal braking event, fluid flows through the non-energized open
apply valves 50, 54, 58, and 62. Additionally, the dump valves 52,
56, 60, and 64 are preferably in their non-energized closed
positions to prevent the flow of fluid to the reservoir 20.
[0023] The brake pedal unit 14 is connected to a brake pedal 70 and
is actuated by the driver of the vehicle as the driver presses on
the brake pedal 70. A brake sensor or switch 72 may be connected to
the ECU 22 to provide a signal indicating a depression of the brake
pedal 70. As will be discussed below, the brake pedal unit 14 may
be used as a back-up source of pressurized fluid to essentially
replace the normally supplied source of pressurized fluid from the
plunger assembly 18 under certain failed conditions of the brake
system 10. The brake pedal unit 14 can supply pressurized fluid in
the conduits 36 and 38 (that are normally closed off at the first
and second isolation valves 30 and 32 during a normal brake apply)
to the wheel brake 12a, 12b, 12c, and 12d as required.
[0024] The brake pedal unit 14 includes a housing having a
multi-stepped bore 80 formed therein for slidably receiving various
cylindrical pistons and other components therein. The housing may
be formed as a single unit or include two or more separately formed
portions coupled together. An input piston 82, a primary piston 84,
and a secondary piston 86 are slidably disposed within the bore 80.
The input piston 82 is connected with the brake pedal 70 via a
linkage arm 76. Leftward movement of the input piston 82, the
primary piston 84, and the secondary piston 86 may cause, under
certain conditions, a pressure increase within an input chamber 92,
a primary chamber 94, and a secondary chamber 96, respectively.
Various seals of the brake pedal unit 14 as well as the structure
of the housing and the pistons 82, 84, and 86 define the chambers
92, 94, and 96. For example, the input chamber 92 is generally
defined between the input piston 82 and the primary piston 84. The
primary chamber 94 is generally defined between the primary piston
84 and the secondary piston 86. The secondary chamber 96 is
generally defined between the secondary piston 86 and an end wall
of the housing formed by the bore 80.
[0025] The input chamber 92 is in fluid communication with the
pedal simulator 16 via a conduit 100, the reason for which will be
explained below. The input piston 82 is slidably disposed in the
bore 80 of the housing of the brake pedal unit 14. An outer wall of
the input piston 82 is engaged with a lip seal 102 and a seal 104
mounted in grooves formed in the housing. A passageway 106 (or
multiple passageways) is formed through a wall of the piston 82. As
shown in FIG. 1, when the brake pedal unit 14 is in its rest
position (the driver is not depressing the brake pedal 70), the
passageway 106 is located between the lip seal 102 and the seal
104. In the rest position, the passageway 106 permits fluid
communication between the input chamber 92 and the reservoir 20 via
a conduit 108. Sufficient leftward movement of the input piston 82,
as viewing FIG. 1, will cause the passageway 106 to move past the
lip seal 102, thereby preventing the flow of fluid from the input
chamber 92 into the conduit 108 and the reservoir 20. Further
leftward movement of the input piston 82 will pressurize the input
chamber 92 causing fluid to flow into the pedal simulator 16 via
the conduit 100. As fluid is diverted into the pedal simulator 16,
a simulation chamber 110 within the pedal simulator 16 will expand
causing movement of a piston 112 within the pedal simulator 16.
Movement of the piston 112 compresses a spring assembly,
schematically represented as a spring 114. The compression of the
spring 114 provides a feedback force to the driver of the vehicle
which simulates the forces a driver feels at the brake pedal 70 in
a conventional vacuum assist hydraulic brake system, for example.
The spring 114 of the pedal simulator 16 can include any number and
types of spring members as desired. For example, the spring 114 may
include a combination of low rate and high rate spring elements to
provide a non-linear force feedback. The simulation chamber 110 is
in fluid communication with the conduit 100 which is in fluid
communication with the input chamber 92. A solenoid actuated
simulator valve 116 is positioned within the conduit 100 to
selectively prevent the flow of fluid from the input chamber 92 to
the simulation chamber, such as during a failed condition in which
the brake pedal unit 14 is utilized to provide a source of
pressurized fluid to the wheel brakes. A check valve 118 in
parallel with a restricted orifice 120 may be positioned with the
conduit 100. The spring 114 of the pedal simulator 16 may be housed
within a non-pressurized chamber 122 in fluid communication with
the reservoir 20 (T1).
[0026] As discussed above, the input chamber 92 of the brake pedal
unit 14 is selectively in fluid communication with the reservoir 20
via a conduit 108 and the passageway 106 formed in the input piston
82. The brake system 10 may include an optional simulator test
valve 130 located within the conduit 108. The simulator test valve
130 may be electronically controlled between an open position, as
shown in FIG. 1, and a powered closed position. The simulator test
valve 130 is not necessarily needed during a normal boosted brake
apply or for a manual push through mode. The simulator test valve
130 can be energized to a closed position during various testing
modes to determine the correct operation of other components of the
brake system 10. For example, the simulator test valve 130 may be
energized to a closed position to prevent venting to the reservoir
20 via the conduit 108 such that a pressure build up in the brake
pedal unit 14 can be used to monitor fluid flow to determine
whether leaks may be occurring through seals of various components
of the brake system 10.
[0027] The primary chamber 94 of the brake pedal unit 14 is in
fluid communication with the second isolation valve 32 via the
conduit 38. The primary piston 84 is slidably disposed in the bore
80 of the housing of the brake pedal unit 14. An outer wall of the
primary piston 84 is engaged with a lip seal 132 and a seal 134
mounted in grooves formed in the housing. One or more passageways
136 are formed through a wall of the primary piston 84. The
passageway 136 is located between the lip seal 132 and the seal 134
when the primary piston 84 is in its rest position, as shown in
FIG. 1. Note that in the rest position the lip seal 132 is just
slightly to the left of the passageway 136, thereby permitting
fluid communication between the primary chamber 94 and the
reservoir 20.
[0028] The secondary chamber 96 of the brake pedal unit 14 is in
fluid communication with the first isolation valve 30 via the
conduit 36. The secondary piston 86 is slidably disposed in the
bore 80 of the housing of the brake pedal unit 14. An outer wall of
the secondary piston 86 is engaged with a lip seal 140 and a seal
142 mounted in grooves formed in the housing. One or more
passageways 144 are formed through a wall of the secondary piston
86. As shown in FIG. 1, the passageway 144 is located between the
lip seal 140 and the seal 142 when the secondary piston 86 is in
its rest position. Note that in the rest position the lip seal 140
is just slightly to the left of the passageway 144, thereby
permitting fluid communication between the secondary chamber 96 and
the reservoir 20 (T2).
[0029] If desired, the primary and secondary pistons 84 and 86 may
be mechanically connected with limited movement therebetween. The
mechanical connection of the primary and secondary pistons 84 and
86 prevents a large gap or distance between the primary and
secondary pistons 84 and 86 and prevents having to advance the
primary and secondary pistons 84 and 86 over a relatively large
distance without any increase in pressure in the non-failed
circuit. For example, if the brake system 10 is under a manual push
through mode and fluid pressure is lost in the output circuit
relative to the secondary piston 86, such as for example in the
conduit 36, the secondary piston 86 will be forced or biased in the
leftward direction due to the pressure within the primary chamber
94. If the primary and secondary pistons 84 and 86 were not
connected together, the secondary piston 86 would freely travel to
its further most left-hand position, as viewing FIG. 1, and the
driver would have to depress the pedal 70 a distance to compensate
for this loss in travel. However, because the primary and secondary
pistons 84 and 86 are connected together, the secondary piston 86
is prevented from this movement and relatively little loss of
travel occurs in this type of failure. Any suitable mechanical
connection between the primary and secondary pistons 84 and 86 may
be used. For example, as schematically shown in FIG. 1, the
right-hand end of the secondary piston 86 may include an outwardly
extending flange that extends into a groove formed in an inner wall
of the primary piston 84. The groove has a width which is greater
than the width of the flange, thereby providing a relatively small
amount of travel between the first and secondary pistons 84 and 86
relative to one another.
[0030] The brake pedal unit 14 may include an input spring 150
generally disposed between the input piston 82 and the primary
piston 84. Additionally, the brake pedal unit 14 may include a
primary spring (not shown) disposed between the primary piston 84
and the secondary piston 86. A secondary spring 152 may be included
and disposed between the secondary piston 86 and a bottom wall of
the bore 80. The input, primary and secondary springs may have any
suitable configuration, such as a caged spring assembly, for
biasing the pistons in a direction away from each other and also to
properly position the pistons within the housing of the brake pedal
unit 14.
[0031] The brake system 10 may further include a pressure sensor
156 in fluid communication with the conduit 36 to detect the
pressure within the secondary pressure chamber 96 and for
transmitting the signal indicative of the pressure to the ECU 22.
Additionally, the brake system 10 may further include a pressure
sensor 158 in fluid communication with the conduit 34 for
transmitting a signal indicative of the pressure at the output of
the plunger assembly 18.
[0032] As shown schematically in FIG. 2, the plunger assembly 18
includes a housing having a multi-stepped bore 200 formed therein.
The bore 200 includes a first portion 202 and a second portion 204.
A piston 206 is slidably disposed within the bore 200. The piston
206 includes an enlarged end portion 208 connected to a smaller
diameter central portion 210. The piston 206 has a second end 211
connected to a ball screw mechanism, indicated generally at 212.
The ball screw mechanism 212 is provided to impart translational or
linear motion of the piston 206 along an axis defined by the bore
200 in both a forward direction (leftward as viewing FIGS. 1 and
2), and a rearward direction (rightward as viewing FIGS. 1 and 2)
within the bore 200 of the housing. In the embodiment shown, the
ball screw mechanism 212 includes a motor 214 rotatably driving a
screw shaft 216. The second end 211 of the piston 206 includes a
threaded bore 220 and functions as a driven nut of the ball screw
mechanism 212. The ball screw mechanism 212 includes a plurality of
balls 222 that are retained within helical raceways 223 formed in
the screw shaft 216 and the threaded bore 220 of the piston 206 to
reduce friction. Although a ball screw mechanism 212 is shown and
described with respect to the plunger assembly 18, it should be
understood that other suitable mechanical linear actuators may be
used for imparting movement of the piston 206. It should also be
understood that although the piston 206 functions as the nut of the
ball screw mechanism 212, the piston 206 could be configured to
function as a screw shaft of the ball screw mechanism 212. Of
course, under this circumstance, the screw shaft 216 would be
configured to function as a nut having internal helical raceways
formed therein. The piston 206 may include structures (not shown)
engaged with cooperating structures formed in the housing of the
plunger assembly 18 to prevent rotation of the piston 206 as the
screw shaft 216 rotates around the piston 206. For example, the
piston 206 may include outwardly extending splines or tabs (not
shown) that are disposed within longitudinally extending grooves
(not shown) formed in the housing of the plunger assembly 18 such
that the tabs slide along within the grooves as the piston 206
travels in the bore 200.
[0033] As will be discussed below, the plunger assembly 18 is
preferably configured to provide pressure to the conduit 34 when
the piston 206 is moved in both the forward and rearward
directions. The plunger assembly 18 includes a seal 230 mounted on
the enlarged end portion 208 of the piston 206. The seal 230
slidably engages with the inner cylindrical surface of the first
portion 202 of the bore 200 as the piston 206 moves within the bore
200. A seal 234 and a seal 236 are mounted in grooves formed in the
second portion 204 of the bore 200. The seals 234 and 236 slidably
engage with the outer cylindrical surface of the central portion
210 of the piston 206. A first pressure chamber 240 is generally
defined by the first portion 202 of the bore 200, the enlarged end
portion 208 of the piston 206, and the seal 230. An annular shaped
second pressure chamber 242, located generally behind the enlarged
end portion 208 of the piston 206, is generally defined by the
first and second portions 202 and 204 of the bore 200, the seals
230 and 234, and the central portion 210 of the piston 206. The
seals 230, 234, and 236 can have any suitable seal structure.
[0034] Although the plunger assembly 18 may be configured to any
suitable size and arrangement, in one embodiment, the effective
hydraulic area of the first pressure chamber 240 is greater than
the effective hydraulic area of the annular shaped second pressure
chamber 242. The first pressure chamber 240 generally has an
effective hydraulic area corresponding to the diameter of the
central portion 210 of the piston 206 (the inner diameter of the
seal 234) since fluid is diverted through the conduits 254, 34, and
243 as the piston 206 is advanced in the forward direction. The
second pressure chamber 242 generally has an effective hydraulic
area corresponding to the diameter of the first portion 202 of the
bore 200 minus the diameter of the central portion 210 of the
piston 206. This configuration provides that on the back stroke in
which the piston 206 is moving rearwardly, less torque (or power)
is required by the motor 214 to maintain the same pressure as in
its forward stroke. Besides using less power, the motor 214 may
also generate less heat during the rearward stroke of piston 206.
Under circumstances in which the driver presses on the pedal 70 for
long durations, the plunger assembly 18 could be operated to apply
a rearward stroke of the piston 206 to prevent overheating of the
motor 214.
[0035] The plunger assembly 18 preferably includes a sensor,
schematically shown as 218, for detecting the position of the
piston 206 within the bore 200. The sensor 218 is in communication
with the ECU 22. In one embodiment, the sensor 218 may detect the
position of the piston 206, or alternatively, metallic or magnetic
elements embedded with the piston 206. In an alternate embodiment,
the sensor 218 may detect the rotational position of the motor 214
and/or other portions of the ball screw mechanism 212 which is
indicative of the position of the piston 206. The sensor 218 can be
located at any desired position.
[0036] The piston 206 of the plunger assembly 18 includes a
passageway 244 formed therein. The passageway 244 defines a first
port 246 extending through the outer cylindrical wall of the piston
206 and is in fluid communication with the secondary chamber 242.
The passageway 244 also defines a second port 248 extending through
the outer cylindrical wall of the piston 206 and is in fluid
communication with a portion of the bore 200 located between the
seals 234 and 236. The second port 248 is in fluid communication
with a conduit 249 which is in fluid communication with the
reservoir 20 (T3). When in the rest position, as shown in FIG. 2,
the pressure chambers 240 and 242 are in fluid communication with
the reservoir 20 via the conduit 249. This helps in ensuring a
proper release of pressure at the output of the plunger assembly 18
and within the pressure chambers 240 and 242 themselves. After an
initial forward movement of the piston 206 from its rest position,
the port 248 will move past the lip seal 234, thereby closing off
fluid communication of the pressure chambers 240 and 242 from the
reservoir 20, thereby permitting the pressure chambers 240 and 242
to build up pressure as the piston 206 moves further.
[0037] Referring back to FIG. 1, the brake system 10 further
includes a first plunger valve 250, and a second plunger valve 252.
The first plunger valve 250 is preferably a solenoid actuated
normally closed valve. Thus, in the non-energized state, the first
plunger valve 250 is in a closed position, as shown in FIG. 1. The
second plunger valve 252 is preferably a solenoid actuated normally
open valve. Thus, in the non-energized state, the second plunger
valve 252 is in an open position, as shown in FIG. 1. A check valve
may be arranged within the second plunger valve 252 so that when
the second plunger valve 252 is in its closed position, fluid may
still flow through the second plunger valve 252 in the direction
from a first output conduit 254 (from the first pressure chamber
240 of the plunger assembly 18) to the conduit 34 leading to the
isolation valves 30 and 32. Note that during a rearward stroke of
the piston 206 of the plunger assembly 18, pressure may be
generated in the second pressure chamber 242 for output into the
conduit 34.
[0038] Generally, the first and second plunger valves 250 and 252
are controlled to permit fluid flow at the outputs of the plunger
assembly 18 and to permit venting to the reservoir 20 (T3) through
the plunger assembly 18 when so desired. For example, the first
plunger valve 250 may be energized to its open position during a
normal braking event so that both of the first and second plunger
valves 250 and 252 are open (which may reduce noise during
operation). Preferably, the first plunger valve 250 is almost
always energized during an ignition cycle when the engine is
running. Of course, the first plunger valve 250 may be purposely
moved to its closed position such as during a pressure generating
rearward stroke of the plunger assembly 18. The first and second
plunger valves 250 and 252 are preferably in their open positions
when the piston 206 of the plunger assembly 18 is operated in its
forward stroke to maximize flow. When the driver releases the brake
pedal 70, the first and second plunger valves 250 and 252
preferably remain in their open positions. Note that fluid can flow
through the check valve within the closed second plunger valve 252,
as well as through a check valve 258 from the reservoir 20
depending on the travel direction of the piston 206 of the plunger
assembly 18.
[0039] It may be desirable to configure the first plunger valve 250
with a relatively large orifice therethrough when in its open
position. A relatively large orifice of the first plunger assembly
250 helps to provide an easy flow path therethrough. The second
plunger valve 252 may be provided with a much smaller orifice in
its open position as compared to the first plunger valve 250. One
reason for this is to help prevent the piston 206 of the plunger
assembly 18 from rapidly being back driven upon a failed event due
to the rushing of fluid through the first output conduit 254 into
the first pressure chamber 240 of the plunger assembly 18, thereby
preventing damage to the plunger assembly 18. As fluid is
restricted in its flow through the relatively small orifice,
dissipation will occur as some of the energy is transferred into
heat. Thus, the orifice should be of a sufficiently small size so
as to help prevent a sudden catastrophic back drive of the piston
206 of the plunger assembly 18 upon failure of the brake system 10,
such as for example, when power is lost to the motor 214 and the
pressure within the conduit 34 is relatively high. As shown in FIG.
2, the plunger assembly 18 may include an optional spring member,
such as a spring washer 277, to assist in cushioning such a rapid
rearward back drive of the piston 206. The spring washer 277 may
also assist in cushioning the piston 206 moving at any such speed
as it approaches a rest position near its most retracted position
within the bore 200. Schematically shown in FIG. 2, the spring
washer 277 is located between the enlarged end portion 208 and a
shoulder 279 formed in the bore 200 between the first and second
portions 202 and 204. The spring washer 277 can have any suitable
configuration which deflects or compresses upon contact with the
piston 206 as the piston 206 moves rearwardly. For example, the
spring washer 277 may be in the form of a metal conical spring
washer. Alternatively, the spring washer 277 may be in the form of
a wave spring. Although the spring washer 277 is shown mounted
within the bore 200 of the plunger assembly 18, the spring washer
277 may alternatively be mounted on the piston 206 such that the
spring washer 277 moves with the piston 206. In this configuration,
the spring washer 277 would engage with the shoulder 279 and
compress upon sufficient rightward movement of the piston 206.
[0040] The first and second plunger valves 250 and 252 provide for
an open parallel path between the pressure chambers 240 and 242 of
the plunger assembly 18 during a normal braking operation. Although
a single open path may be sufficient, the advantage of having both
the first and second plunger valves 250 and 252 is that the first
plunger valve 250 may provide for an easy flow path through the
relatively large orifice thereof, while the second plunger valve
252 may provide for a restricted orifice path during certain failed
conditions (when the first plunger valve 250 is de-energized to its
closed position.
[0041] During a typical or normal braking operation, the brake
pedal 70 is depressed by the driver of the vehicle. In a preferred
embodiment of the brake system 10, the brake pedal unit 14 includes
one or more travel sensors 270 (for redundancy) for producing
signals transmitted to the ECU 22 that are indicative of the length
of travel of the input piston 82 of the brake pedal unit 14.
[0042] During normal braking operations, the plunger assembly 18 is
operated to provide pressure to the conduit 34 for actuation of the
wheel brakes 12a, 12b, 12c, and 12d. Under certain driving
conditions, the ECU 22 communicates with a powertrain control
module (not shown) and other additional braking controllers of the
vehicle to provide coordinated braking during advanced braking
control schemes (e.g., anti-lock braking (AB), traction control
(TC), vehicle stability control (VSC), and regenerative brake
blending). During a normal brake apply, the flow of pressurized
fluid from the brake pedal unit 14, generated by depression of the
brake pedal 70, is diverted into the pedal simulator 16. The
simulator valve 116 is actuated to divert fluid through the
simulator valve 116 from the input chamber 92. Note that the
simulator valve 116 is shown in its energized state in FIG. 1.
Thus, the simulator valve 116 is a normally closed solenoid valve.
Also note that fluid flow from the input chamber 92 to the
reservoir 20 is closed off once the passageway 106 in the input
piston 82 moves past the seal 104.
[0043] During the duration of a normal braking event, the simulator
valve 116 remains open, preferably. Also during the normal braking
operation, the isolation valves 30 and 32 are energized to
secondary positions to prevent the flow of fluid from the conduits
36 and 38 through the isolation valves 30 and 32, respectively.
Preferably, the isolation valves 30 and 32 are energized throughout
the duration of an ignition cycle such as when the engine is
running instead of being energized on and off to help minimize
noise. Note that the primary and secondary pistons 84 and 86 are
not in fluid communication with the reservoir 20 due to their
passageways 136 and 144, respectively, being positioned past the
lip seals 132 and 140, respectively. Prevention of fluid flow
through the isolation valves 30 and 32 hydraulically locks the
primary and secondary chambers 94 and 96 of the brake pedal unit 14
preventing further movement of the primary and secondary pistons 84
and 86.
[0044] It is generally desirable to maintain the isolation valves
30 and 32 energized during the normal braking mode to ensure
venting of fluid to the reservoir 20 through the plunger assembly
18 such as during a release of the brake pedal 70 by the driver. As
best shown in FIG. 1, the passageway 244 formed in the piston 206
of the plunger assembly 18 permits this ventilation.
[0045] During normal braking operations, while the pedal simulator
16 is being actuated by depression of the brake pedal 70, the
plunger assembly 18 can be actuated by the ECU 22 to provide
actuation of the wheel brakes 12a, 12b, 12c, and 12d. The plunger
assembly 18 is operated to provide desired pressure levels to the
wheel brakes 12a, 12b, 12c, and 12d compared to the pressure
generated by the brake pedal unit 14 by the driver depressing the
brake pedal 70. The electronic control unit 22 actuates the motor
214 to rotate the screw shaft 216 in the first rotational
direction. Rotation of the screw shaft 216 in the first rotational
direction causes the piston 206 to advance in the forward direction
(leftward as viewing FIGS. 1 and 2). Movement of the piston 206
causes a pressure increase in the first pressure chamber 240 and
fluid to flow out of the first pressure chamber 240 and into the
conduit 254. Fluid can flow into the conduit 34 via the open first
and second plunger valves 250 and 252. Note that fluid is permitted
to flow into the second pressure chamber 242 via a conduit 243 as
the piston 206 advances in the forward direction. Pressurized fluid
from the conduit 34 is directed into the conduits 40 and 42 through
the isolation valves 320 and 322. The pressurized fluid from the
conduits 40 and 42 can be directed to the wheel brakes 12a, 12b,
12c, and 12d through open apply valves 50, 54, 58, and 62 while the
dump valves 52, 56, 60, and 64 remain closed. When the driver lifts
off or releases the brake pedal 70, the ECU 22 can operate the
motor 214 to rotate the screw shaft 216 in the second rotational
direction causing the piston 206 to retract withdrawing the fluid
from the wheel brakes 12a, 12b, 12c, and 12d. The speed and
distance of the retraction of the piston 206 is based on the
demands of the driver releasing the brake pedal 70 as sensed by the
sensor 218. Under certain conditions, the pressurized fluid from
the wheel brakes 12a, 12b, 12c, and 12d may assist in back-driving
the ball screw mechanism 212 moving the piston 206 back towards its
rest position.
[0046] In some situations, the piston 206 of the plunger assembly
18 may reach its full stroke length within the bore 200 of the
housing and additional boosted pressure is still desired to be
delivered to the wheel brakes 12a, 12b, 12c, and 12d. The plunger
assembly 18 is a dual acting plunger assembly such that it is
configured to also provide boosted pressure to the conduit 34 when
the piston 206 is stroked rearwardly (rightward) or in a reverse
direction. This has the advantage over a conventional plunger
assembly that first requires its piston to be brought back to its
rest or retracted position before it can again advance the piston
to create pressure within a single pressure chamber. If the piston
206 has reached its full stroke, for example, and additional
boosted pressure is still desired, the second plunger valve 252 is
energized to its closed check valve position. The first plunger
valve 250 is de-energized to its closed position. The electronic
control unit 22 actuates the motor 214 in a second rotational
direction opposite the first rotational direction to rotate the
screw shaft 216 in the second rotational direction. Rotation of the
screw shaft 216 in the second rotational direction causes the
piston 206 to retract or move in the rearward direction (rightward
as viewing FIGS. 1 and 2). Movement of the piston 206 causes a
pressure increase in the second pressure chamber 242 and fluid to
flow out of the second pressure chamber 242 and into the conduit
243 and the conduit 34. Pressurized fluid from the conduit 34 is
directed into the conduits 40 and 42 through the isolation valves
30 and 32. The pressurized fluid from the conduits 40 and 42 can be
directed to the wheel brakes 12a, 12b, 12c, and 12d through the
opened apply valves 50, 54, 58, and 62 while dump valves 52, 56,
60, and 64 remain closed. In a similar manner as during a forward
stroke of the piston 206, the ECU 22 can also selectively actuate
the apply valves 50, 54, 58, and 62 and the dump valves 52, 56, 60,
and 64 to provide a desired pressure level to the wheel brakes 12a,
12b, 12c, and 12d, respectively. When the driver lifts off or
releases the brake pedal 70 during a pressurized rearward stroke of
the plunger assembly 18, the first and second plunger valves 250
and 252 are preferably operated to their open positions, although
having only one of the valves 250 and 252 open would generally
still be sufficient. Note that when transitioning out of a slip
control event, the ideal situation would be to have the position of
the piston 206 and the displaced volume within the plunger assembly
18 correlate exactly with the given pressures and fluid volumes
within the wheel brakes 12a, 12b, 12c, and 12d. However, when the
correlation is not exact, fluid can be drawn from the reservoir 20
via the check valve 258 into the chamber 240 of the plunger
assembly 18.
[0047] During a braking event, the ECU 22 can selectively actuate
the apply valves 50, 54, 58, and 62 and the dump valves 52, 56, 60,
and 64 to provide a desired pressure level to the wheel brakes,
respectively. The ECU 22 can also control the brake system 10
during ABS, DRP, TC, VSC, regenerative braking, and autonomous
braking events by general operation of the plunger assembly 18 in
conjunction with the apply valves and the dump valves. Even if the
driver of the vehicle is not depressing the brake pedal 70, the ECU
22 can operate the plunger assembly 18 to provide a source of
pressurized fluid directed to the wheel brakes, such as during an
autonomous vehicle braking event.
[0048] In the event of a loss of electrical power to portions of
the brake system 10, the brake system 10 provides for manual push
through or manual apply such that the brake pedal unit 14 can
supply relatively high pressure fluid to the conduits 36 and 38.
During an electrical failure, the motor 214 of the plunger assembly
18 might cease to operate, thereby failing to produce pressurized
hydraulic brake fluid from the plunger assembly 18. The isolation
valves 30 and 32 will shuttle (or remain) in their positions to
permit fluid flow from the conduits 36 and 38 to the wheel brakes
12a, 12b, 12c, and 12d. The simulator valve 116 is shuttled to its
closed position to prevent fluid from flowing out of the input
chamber 92 to the pedal simulator 16. During the manual
push-through apply, the input piston 82, the primary piston 84, and
the secondary piston 86 will advance leftwardly such that the
passageways 106, 136, 144 will move past the seals 102, 132, and
140, respectively, to prevent fluid flow from their respective
fluid chambers 92, 94, and 96 to the reservoir 20, thereby
pressurizing the chambers 92, 94, and 96. Fluid flows from the
chambers 94 and 96 into the conduits 38 and 36, respectively, to
actuate the wheel brakes 12a, 12b, 12c, and 12d.
[0049] The inherent design of the brake system 10 allows the ECU
22, with the assistance of sensors, to be able to detect various
braking conditions and alert the driver accordingly. For example,
the brake system 10 could be configured to detect the presence of
brake fade and alter the brake pedal feel to alert the driver that
something is amiss. For example, the brake pedal feel could be
altered such that a greater depression travel of the brake pedal 70
is required by the driver's foot to obtain the same braking force
during a non-brake fade event. This alteration of the brake pedal
feel can alert the driver that the brake system 10 is undergoing an
issue, such as brake fade.
[0050] Brake fade can be defined as a temporary reduction of
braking power within the wheel brakes of the vehicle. Brake fade
typically occurs when the brake pad and the brake rotor no longer
generate sufficient mutual friction therebetween due to a buildup
of heat. Obviously, this results in unexpected brake system
behavior. Generally, brake pads are designed to work within certain
operating temperatures and if used outside this operating window
the corresponding wheel brake will underperform. This abnormally
high heat can be generated during various conditions such as high
vehicle load conditions, high speed conditions, or steep alpine
descent conditions. Since brake fade occurs when the brake pads are
overheated, this phenomenon is usually only temporary and braking
performance typically returns once the brakes have cooled down.
During the brake fade event, however, it is generally desirable to
alert the driver of this condition so that they can act accordingly
and to later help reduce the buildup of heat within the wheel
brakes.
[0051] With respect to the brake system 10, a different strategy
for detecting and alerting the driver of brake fade should be taken
into consideration compared to a conventional brake system. For
example, in the brake system 10, direct feedback through the brake
pedal 70 of the health of the brake system 10 may be altered,
masked, or even non-existent. The overall brake pedal travel in
normal operation of the brake system 10 is significantly reduced
when compared to conventional brake systems.
[0052] In conventional brake systems, the driver can gain
information about brake fade by experiencing a change in the pedal
force/travel/deceleration relationship. The driver then has the
ability to compensate directly by applying more force and adjusting
to a longer pedal travel up to their capability. However, with the
brake system 10, it is possible that the brake system 10 may incur
(1) a loss of output and (2) an increase in compliance during faded
braking if left unchecked, i.e., if the brake system 10 is not
controlled to compensate for the detection and change in driver
pedal feel behavior. A loss of output of braking pressure can cause
additional pedal force and travel such that the driver will push
deeper into the pedal stroke to command a higher pressure to
achieve their desired vehicle deceleration. Since the force/travel
characteristic of the brake system 10 is fixed, the pedal will
remain "firm" for the driver. The driver may continue to push
deeper until the end of the pedal travel is reached, whereby
maximum pressure will then be commanded. Since the brake system 10
is preferably a closed loop system with respect to pressure, the
brake system 10 will eventually compensate for the compliance
increase. Since there is a fixed fluid volume available before the
brake system 10 needs to be replenished, the driver will experience
more replenishments if the compliance growth is large enough during
advanced braking events such as ABS. This compliance increase may
be of concern since there would be almost no direct pedal feel
difference to the driver. Thus, it is desirable to provide a
comprehensive strategy to handle performance of the brake system 10
and to give a desired warning.
[0053] Although a brake thermal model exists for use in
conventional brake systems, it does not properly comprehend
racetrack use or trailer towing if used as sole means to provide
the driver notification. Therefore, it is preferred that brake
output and system compliance are constantly monitored along with
the brake thermal model, to detect faded brakes. There are
generally three major components for the brake fade strategy with
respect to the brake system 10: (1) Adaptive References, (2) Brake
Fade Factors, and (3) Driver Notification and Mitigation.
[0054] With respect to adaptive references, it is preferred that
pressure output (Output) references and Pressure-Volume (PV)
references are established for the monitoring mechanism to compare
the performance of the brake system 10 against. The characteristics
of the brake system 10 can change during the life of the vehicle
for multiple reasons. Thus, fixed and nominal references should not
be used to monitor for a significant change in Output or PV caused
by faded brakes. Therefore, adaptive references maps should be
established and utilized. In a preferred operation, the maps will
be adjusted after each ignition cycle if the thermal model confirms
that the brake temperature is within normal operation range. At
sufficiently high temperatures, the map learning will be stopped
and the last known good map will be used as reference for this
ignition cycle.
[0055] In a preferred method, the reference maps are established by
storing vehicle deceleration and boost system volume at different
pressure points into a histogram. The pressure points are
pre-defined in the trims. They are preferably picked to cover a
wide pressure range during braking. For example, the points may be
10 bar, 15 bar, 20 bar, 25 bar, 30 bar, 35 bar, 50 bar, and 100
bar. This can be referred to as an eight points reference table. If
the learning conditions (as discussed below) are satisfied, every
time there is boosted brake pressure, a searching algorithm looks
for the pre-defined pressure points. If the current boost pressure
is within the range of .+-.0.2 bar of one of the points, the
current vehicle deceleration or volume data is learned to its map
accordingly. To provide an accurate reference and shorten the
learning period, the last known reference map is preferably stored
in non-volatile random-access memory (NVRAM), and will be used as
the starting point for each new ignition cycle. For new vehicles
after manufacture, there may be one set of default reference trims
for each map to start with.
[0056] One possible drawback of choosing the learning pressure
points so widely is that some points can almost never be learned,
such as for example, the 100 bar point. The reference will become
less accurate at high pressure. Fortunately, it has been found that
test data shows that the maps only shift up or down without much of
offset variations across the whole pressure range when temperature
increases. This characteristic enables projection to the adjusted
reference for the un-learned pressure points. The algorithm
calculates the offset between the learned value and the default
reference value for each pressure point. The algorithm then weighs
all of the offsets and adds them together as the final total
offset. The more times one point has been learned, the more its
offset weighs into the equation. In one example, the point 15 bar
has been learned 20 times, and each of the other points have been
learned 10 times. The 15 bar offset is 2, and the others are 1.
Then the total times learned is 10.times.7+20=90. The 15 bar point
weighs 20/90.times.100%=22.22%, and each other point weighs
10/90.times.100%=11.11%. Therefore, the final offset can be decided
as 2.times.22.22%+1.times.11.11%.times.7=1.22. The final reference
for those points that are never learned will then be shifted by
1.22.
[0057] The functionality of the histogram represents a special kind
of filter. It is especially suitable for slow learning algorithms,
because the latest value isn't used as the most reliable. For
example, there is illustrated in FIG. 3 a figure or picture of a
histogram. The main data of the histogram is the array shown in
FIG. 3. FIG. 3 also illustrates how the given histogram trims
(number of cells, min and max value) are interpreted. For brake
fade reference learning, the importance is the average value of the
histogram data, calculated with the equation:
x _ = 1 n i = 1 n x i ##EQU00001##
[0058] With respect to creating a PV Reference table, there are
various conditions that preferably should be first met. These can
be considered as enable conditions. One of those conditions is that
the pressure gradient should be less than about 30 bar/s. The
reference should not be established while the brake system 10 is
undergoing a dynamic event. For example, this may not be desirable
due to orifice effects within the brake system 10. A relatively
large pressure gradient is a good indicator that the brake system
10 is undergoing a dynamic event. Another condition which should be
first met prior to creating a PV reference table is that the
pressure gradient should be greater than about -3 bar/s. The
characteristic of the brake system 10 can make it difficult to
calculate volume during a pressure release. Additionally, the slip
control should not be active. Under slip control, the dump valves
52, 56, 60, 64 may open and the fluid volume that is diverted into
the reservoir 20 can be hard to model. It is also generally not
necessary to learn PV values during such events. Another condition
that should be met is that the wheel brake temperature should be
less than about 250 C. The PV characteristic changes along with
brake temperature. It is preferred that reference should be only
established under normal operating temperature. Another condition
is that brake fade should not be detected.
[0059] Once learning is enabled, the eight points reference table
pulls the average values from the histogram, and stores them in
NVRAM. In the meantime, a PV threshold is established. Now, with
given boost pressure, the fluid volume needed to generate this
pressure can be looked up from the reference table. This volume
then becomes the volume threshold.
[0060] With respect to creating a Deceleration Reference table,
there are also various conditions that preferably should be first
met. One of those conditions is that vehicle speed should be faster
than about 4 m/s. The pressure gradient should also be less than
about 30 bar/s since the reference should not be established while
the vehicle or the brake system is under a dynamic event. The
pressure gradient should be greater than about -3 bar/s. Vehicle
deceleration to pressure relationship is not accounted for during a
pressure release. Additionally, slip control should not be active.
Also, boost pressure should be larger than about 15 bar. There
should be some pressure already in the system before enabling the
learning to avoid the initial pitch moment of the vehicle at the
beginning of a braking event. Additionally, the wheel brake
temperature should be less than about 250 C. Another condition is
that brake fade should not be detected.
[0061] Similar to the PV reference table, a pressure to
deceleration reference table is established during each braking
events. Now, with a given boost pressure, the deceleration
threshold can be looked up from the reference table. The
deceleration data used can be termed "decel generated by brake".
This "decel generated by brake" signal is provided by the brake
system 10 such as by the ECU 22 for ABS purposes. This eliminates
the factors such as tire rolling resistance, air drag, engine drag,
etc. to calculate the deceleration that is only generated by the
brake system. One of the benefits of using this signal is to take
the noise factors out of the equation and establish reliable
reference between pressure and deceleration.
[0062] With respect to brake fade factors, the brake system
performance degradation caused by fade is represented by a fade
factor in a percentage value. There are generally two components
that contribute to the overall fade factor. The first component
being output, and the second component being compliance. Each one
calculates their own fade factor by comparing the current
performance against the reference maps. The two factors will be
blended in as the overall fade factor. The PV factor measures the
fluid volume increase during braking events, and calculates a
change factor in percentage, which is calculate by:
Factor=((Measured volume-Volume thr)/Max_error_thr)*100%. The
"Measured volume" is the volume of fluid that is currently
consumed. The "Volume thr" relates to the volume threshold as
explained above. The "Max_error_thr" is the allowed maximum PV
growth during high temperature. This is trimmed such that the
factor is 100% when the front rotor temperature is about 600 C.
Obviously with the equation, the PV factor can exceed 100%. There
is no upper limit, so that it can be used to monitor brake fluid
boil, as discussed below.
[0063] Referring now to FIG. 4, an example of a PV factor
calculation will now be discussed. To calculate the PV factor at
400 C, a 40 bar pressure can be used as an example. According to
the equation above: PV Factor=((11-7.5)/7)*100%=50%.
[0064] The mechanism used to calculate deceleration factor is
preferably the same as for the PV factor, with the addition that
the deceleration factor is limited to 100%. There is preferably a
low pass filter on the deceleration factor to eliminate noise
caused by suspension movement, so that only steady state is
considered.
[0065] If the brake rotor and pad temperature stay high for a long
period of time, the heat could be introduced or soaked into the
calipers and eventually boil the fluid. When this happens, the
compliance of the brake system 10 would have a dramatic increase.
The plot of FIG. 5 illustrates the PV characteristics of the brake
system 10 under different conditions. The detection for brake fluid
boil generally has two criteria. The first criteria being that the
brake temperature should be higher than the minimum fade enable
temperature, which has been found to be about 400 C. The second
criteria is that the PV factor should remain greater than 135% for
about 1 second. Based on the data in the plot of FIG. 5, the PV
factor can be calculated during fluid boiling to be about 140%.
High brake temperature with a dramatic system compliance increase
indicates fluid boil. Once fluid boil is detected, the brake system
10 can be determined to be under a brake fade "stage 2" (as
discussed below). In this stage, it is preferred to limit the
vehicle's top speed.
[0066] The brake thermal model estimates the brake rotor
temperature by considering factors such as vehicle speed, brake
pressure, ambient temperature, etc. The highest average axle
temperature is used as the minimum enable threshold (about 400 C)
for brake fade detection.
[0067] The overall brake fade factor is what finally triggers fade
detection. The PV factor and the deceleration factor are weighted
and then added together to determine the final brake fade factor in
percentage form to represent the fade level and the performance
changes. Typically, the higher the overall factor is, the worse the
brake fade is. The overall brake fade factor is calculated with the
equation: Factor=A*PV factor+(100%-A)*Decel factor. "A" is the
weighing constant measured in percentage form. This constant
depends on the material property of the brake pads. For example, in
some brake pads, as the temperature increases, the output loss
could be dramatic. While other brake pad materials may endure a
consistent output, but a relatively large increase in compliance.
Preferably, the factor is calibrated such that whichever has more
of an impact on the brake performance is weighted more. For
example, on a large truck or SUV platform, the compliance may
consistently increase with growing temperature, while the output
barely changes. In this case, "A" could be calibrated to 85% for
final fade factor calculation.
[0068] Brake fade is not something that generally needs to be
detected as quickly as possible. The phenomena develops very slowly
and also typically disappears slowly. Preferably, the detection is
only enabled when both the brake system 10 and the vehicle are in a
steady state. It is preferred that certain conditions are met to
enable detection. These conditions include, for example, that the
vehicle speed is above about 4 m/s and that the brake system 10 is
not undergoing a slip control event. Additionally, the pressure
gradient should be less than about 300 bar/s and greater than about
-3 bar/s. Also, the boost pressure should be greater than about 15
bar/s.
[0069] The term "Stage 1" brake fade can be defined as when the
brakes are hot and that there are signs of performance degradation
due to the high temperature, but not significantly decreased, and
there is very little possibility of hardware damage. In addition to
the overall brake fade detection enable conditions discussed above,
the entry criteria for Stage 1 brake fade should include that the
maximum axle average brake thermal temperature exceeds about 400 C
and that the brake fade factor is above 45% for about 800
milliseconds. Of course, "Stage 2" brake fade should not have been
reached.
[0070] The term "Stage 2" brake fade can be defined as when the
brake system is overheated and the performance has degraded
significantly. Hardware is likely to be damaged if the brake system
stays in this current condition for a period of time. Therefore, a
countermeasure should be in place in a relatively short period of
time upon determination that the Stage 2 has been reached. The
entry criteria for Stage 2 brake fade should include that the
maximum axle average brake thermal temperature exceeds about 400 C
and that the brake fade factor is above 90% for about 800
milliseconds. If brake fluid boil is detected, the above criteria
may be overwritten and Stage 2 brake fade will be triggered
immediately.
[0071] The brake fade stages may be reset upon determination of
certain conditions being met. For example, the brake fade stages
may be reset if the maximum axle average brake thermal temperature
falls below 250 C. Alternatively or additionally, the brake fade
stages may be reset if the PV factor is less than about 40% and/or
the Deceleration factor is less than about 30%, for example. After
500 milliseconds within these conditions, the fade detection can be
reset. However, a special situation may be during a cycling
ignition. If the driver stopped the vehicle and had an ignition
cycle after any stage brake fade, the fade detection may
automatically start the new ignition cycle with a Stage 1 fade. In
the case of a Stage 2 brake fade, always limiting vehicle speed may
be considered unsafe in some situations, such as during highway
driving. However, the system could be configured to be capable of
going back to Stage 2 if the above discussed criteria match even in
the new ignition cycle.
[0072] Upon brake fade detection, it is generally advisable to
notify the driver of this condition and to possibly control the
brake system 10 accordingly. Under faded conditions, the intent is
to provide as much conventional feedback of brake fade to the
driver as possible within the constraints of the brake system 10.
After crossing both fade factor and thermal thresholds, the ECU 22
of the brake system 10 can notify the driver that they are reaching
the limits of the brake system through a driver information center
(DIC) display message, for example. Additionally, the brake system
10 could alter the pedal feel to alert the driver. Optionally, if
the driver fails to take action to reduce the input to the brakes,
the vehicle could be speed limited to force reduction as a last
resort.
[0073] At detection of a Stage 1 fade, the driver should be warned
and the brake pedal travel to pressure curve is altered to simulate
the "fade feel" as in a conventional brake systems such that the
brake pedal 70 needs to be pushed further to achieve the same
amount of pressure. There may also be a DIC message with an
optional chime or other audible warning to further alert the
driver. There is illustrated in FIG. 6 an example of a brake pedal
feel curve showing a comparison between a normal pedal travel to
pressure curve and a faded pedal travel to pressure curve. While at
a detection of a Stage 2 fade, the brake pedal travel to pressure
curve remains altered by the brake system 10, and a reminder DIC
message may be sent. In addition, vehicle speed may be limited to a
maximum speed, such as 70 kph, for example.
[0074] With respect to the various valves of the brake system 10,
the terms "operate" or "operating" (or "actuate", "moving",
"positioning") used herein (including the claims) may not
necessarily refer to energizing the solenoid of the valve, but
rather refers to placing or permitting the valve to be in a desired
position or valve state. For example, a solenoid actuated normally
open valve can be operated into an open position by simply
permitting the valve to remain in its non-energized normally open
state. Operating the normally open valve to a closed position may
include energizing the solenoid to move internal structures of the
valve to block or prevent the flow of fluid therethrough. Thus, the
term "operating" should not be construed as meaning moving the
valve to a different position nor should it mean to always
energizing an associated solenoid of the valve.
[0075] The principle and mode of operation of this invention have
been explained and illustrated in its preferred embodiment.
However, it must be understood that this invention may be practiced
otherwise than as specifically explained and illustrated without
departing from its spirit or scope.
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